Storage and retrieval systems were originally designed as unit loading devices for large pallets. Over the years, these systems have undergone many changes. One change, in particular, has been the shift from large-load stackers to mini-stackers and micro-stackers.
The smaller storage and retrieval systems (i.e., those employing mini-stackers or micro-stackers) are especially well suited for factory environments, as well as for warehouse environments. In addition, these systems are used for work-in-process buffers at the point of use to meet just-in-time and other processing requirements.
With the advent of the computer, sophisticated control systems have been developed for automating the operation of storage and retrieval systems. Thus, modern storage and retrieval systems can be automatically controlled by a computer to provide random access to numerous stock-keeping units. Some automated storage and retrieval systems even have capability to fill orders remotely.
A typical automated storage and retrieval system includes a single-level or multi-level rotary rack having a plurality of bins, each bin being adapted to receive a product or a product container. The single-level rotary rack is employed when the storage retrieval or random access rate at the bins is subordinate to storage volume at the bins. This type of rack rotates all of the bins simultaneously in the same direction. In contrast, the bins of the multi-level rotary rack are arranged in rows at different levels of the rack with each level being independently driven and controlled. This system is well suited for high rates of random access, or high throughput rates, at the bins.
An extractor/inserter device interfaces with either the single-level rotary racks or the multi-level rotary racks for the purpose of performing automated pick and put operations, whereby products or product containers are transferred to and from a point of need, such as a conveyor interface, operator work station or robotic interface. Traditionally, these extractor/inserter devices have been equipped with a pair of pinchers adapted to grip a product or a product container by way of a mechanical clamping operation.
While the extractor/inserter devices equipped with pinchers have performed well in applications in which the product or product containers are made from a standard material and in relatively standard shapes and sizes, they do encounter difficulties when used in certain applications in which the product or product containers are made from a variety of materials and come in different sizes and shapes. The U.S. Postal Service's mail tray staging and retrieval system is one example of the latter type of application. More particularly, because postal trays are fabricated from as many as three different materials and come in a variety of sizes and shapes having sidewalls which are angled to varying degrees, they make positive gripping by the pincher-type extractor/inserter devices difficult, if not impossible.